Frequency modulated altimeter or distance indicator



May 6, 1947 R. c. SANDERS, JR 2,420,199

FREQUENCY MODULATED ALTIMETER OR DISTANCE INDICATOR Filed June 4, 1942Fr-@amar Qs@ 155W @dcnfandcra Jr.'

Gttorneg Patented May 6, 1947 ZAZQJQQ FREQUENCY MODULATED ALTEVIETER ORDISTANCE INDICATOR Royden C. Sanders, Jr., Haddonfield, N. J., as

signor to Radio Corporation of America, a corporation of DelawareApplication June 4, 1942, Serial No. 445,720

9 Claims.

My invention relates to radio systems such as radio altimeters forindicating distance or height and Will be described particularly withrespect to systems wherein the radiated wave is frequency modulated.

Systems of this character are described in Bentley Patent No. 2,011,392and Espensch-ied Patent 2,645,071. In these systems, the radiatedfrequency modulated wave is reflected from the earths surface or otherobject and the refiected wave is received in a heterodyne receiverlocated in the vicinity of the transmitter. The heterodyning or mixingsignal for the receiver is obtained directly from the transmitterwhereby the receiver output includes a signal of the beat or differencefrequency which frequency is determined by the time required for .theradiated signal to reach the reflecting object and return to thereceiver.

It has been found that one of the things that limits the full use of thesensitivity of the receiver is the interfering signal resulting from theamplitude modulation normally prese-nt on the mixing or heterodyningsignal applied to the heterodyne detector. When the optimum mixingVoltage, i. e., optimum with respect to noise ratio, is introduced onthe heterodyne detector, the amplitude modulated` signal in the outputof the receiver is stillten to fifty times the noise level when thetransmitter signal is swept through a` twenty megacycle bandwidth.

'it might be expected that there would be no amplitude modulation on themixing signal. This would be true if it were not for the diiiiculty oftransmitting a wide frequency band signal over a line with uniformattenuation at all frequencies. In actual practice there are reflectionson the line to the transmitter antenna `and on the line between thetransmitter and the receiverl because of practically unavoidableimperfect line terminations, for example.

While amplitude modulation on the mixing signal causes the greater partof the above mentioned interference, there also may be someinterference, usually almost negligible, caused by amplitude modulationon the received reflected signal. The latter modulation is introducedbecause of such factors as imperfect termination of the line from thetransmitter to the transmitter antenna and possibly because of frequencyselective renection. of the signal.

An object of my invention is to provide improved means for reducing theinterfering amplitude modulation signal appearing in the output of areceiver in a system of the above described type, A further object ofthe invention is to provide an improved receiver for a system of theabove described type.

In a preferred embodiment of the invention, a balanced diode detector isemployed in which each detector tube has the reflected signal and (Cl.Z50-1.68)

2. the heterodyning signalapplied thereto with the two signals having aphase relation with respect to each other at one tube that is theopposite of their phase relation with respect to each other at the othertube. The reflected signal may be applied to the tubes in parallelrelation while the heterodyning signal is applied to them in push-pullrelation, for example. By taking the output signal off the anode of onetube and olf the cathode of the other tube and properly combining thetwo output signals, any amplitude modulation of the applied signals, andparticularly of the heterodyning signal, may be balanced out while theheterodyne or difference frequency signal appears in the detector outputcircuit. Likewise, any interfering or jamming sig-I nal may be balancedout.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing in which:

Figure l isl a circuit and block diagram of one embodiment of theinvention;

Figure 2 is a :circuit diagram of a receiver -designed in accordancewith a preferred embodiment of the invention; and

Figure 3 is a view showing the physical ary rangement of a portion ofthe receiver illustrated in Fig. 2.

In the several figures, like parts are indicated by the same referencecharacters.

Referring to Fig. 1, the invention is shown applied to a radio altimetercomprising a frequency modulated radio transmitter Ill which radiates afrequency modulated carrier wave from an antenna H. Suitable means forsweeping the carrier wave frequency through a certain frequency rangeperiodically is indicated at I2.

The associated receiver comprises a pair of diodes I3 and I4 which havesignal applied thereto from the secondary I6 of a transformer I'I, oneend of which is connected directly to the anode of diode I4- and theother end of which is connected through a capacitor I8 to the anode ofdiode i3. The midpoint of secondary I5 is connected to ground throughthe secondary I9 of a transformer 2I. The cathode of diode I3 isconnected directly to ground While the cathode of diode IB is connectedto ground through a blocking capacitor 22.

The reected signal is picked up by a receiver antenna 23 and is suppliedthrough a coaxial line to the primary 24 of transformer 2|. Thus thereflected signal is applied with the same polarity to the anodes ofdiodes I3 and I4. Stated differently, it is applied in parallel relationto the diodes.

The same frequency modulated signal that is supplied to the transmitterantenna I I is also supplied through a coaxial line 26 to the primary 21of transformer II as a heterodyning signal whereby a difference or beatfrequency signal is produced which is a measure of the time required forthe signal radiated from antenna II to reach antenna 23 by reflectionand, therefore, a measure of the distance to the reflecting surface. Itwill be seen that the heterodyning signal is applied with oppositepolarity to the diodes I3 and I4, i. e., it is applied in push-pullrelation instead of in parallel relation.

The detector push-pull input circuit is tuned to the mid-frequency ofthe heterodyning signal by a variable capacitor 28. This circuit may betraced from the center point of winding I6 through the upper half ofthis winding, through capacitor 28 and through the lower half of windingI6 back to the center point. The two parallel input circuits are tunedto the said mid-frequency by selecting proper inductance values,particularly for coil |9. The latter circuits, which are tuned in partby the diode capacities indicated at 29 and 38, may be traced asfollows: One circuit from ground through coil I9, the upper part of coilI8 and through the diode I3 to ground, and the other circuit from groundthrough coil I9, the lower part of coil I6 and through the diode I4 andthe blocking capacitor 22 to ground.

The beat or difference frequency output signal is taken off the anode ofdiode I3 and off the cathode of diode I4 through radio frequency .chokecoils 3| and 32, respectively, and applied to opposite ends of abalancing resistor 33. A variable tap 36 is connected to one end of acomparatively high resistance resistor 34 which may be the grid leak ofthe following audio amplifier tubes. The other end of resistor 34 isgrounded.

Any undesired amplitude modulation on the reflected signal or on theheterodyning signal may be balanced out by setting the tap 36 at theproper point since the envelope of any amplitude modulation has the samepolarity at the anode of diode I3 as at the anode of diode I4 and sincethe audio output of diode I4 is reversed with respect to that of diodeI3.

The desired beat frequency signal, on the other hand, will not balanceout because the beat frequency signal at the anode of diode I3 is 180degrees out of phase with the beat frequency signal of diode I4.Therefore, when these two signals are reversed in relative polarity bytaking one signal off the anode and the other signal o the cathode, thetwo signals add in the resistor 34,

It will be seen from the foregoing that the invention is based upon thefact that if a heterodyning signal of one polarity is mixed with areceived signal to produce a, beat frequency signal and if this sameheterodyning signal, but of opposite polarity, is mixed with thereceived signal to produce another beat frequency signal, then the twobeat frequency signals are of opposite polarity, i. e., 180 degrees outof phase. It is also based upon the fact that the detected amplitudemodulation signal, i. e., the amplitude modulation envelope, is of thesame polarity at the anodes of the two dio-des.

In Fig. 2 there is shown schematically a preferred embodiment of theinvention which operates in a manner similar to that of Fig. 1. Itcomprises a balanced diode detector which includes the diodes 16 and 11having their anodes 18 and 19 connected to a tuned line 8|. This lineincludes two coaxial conductors 82 and 83, the inner conductors of whichare connected to the anodes of the diodes 16 and 11. The outerconductors of the coaxial conductors are connected at each end to theinner conductor by means of capacitors 84, 86, 81 and 88 whereby theinner and outer conductors are at the same potential at radiofrequencies.

The cathode of diode 16 is conductively connected to ground preferablythrough a low impedance resistor 88 and a capacitor 9| in parallel. Thecathode of diode 11 is connected to ground at radio frequencies througha capacitor 92. The cathode 88 may be connected directly to ground, ifdesired, but the connection through capacitor 9| is preferred since itmakes the circuit more symmetrical and more easily balanced. This istrue because capacitor 9| at diode 18 corresponds to capacitor 82 atdiode 11, the latter capacitor being required where the beat frequencysignal is taken off the cathode, The resistor 89 is required to completethe direct current path for diode 16. It may have a resistance of 108ohms, for example. Each side of the heater filament for cathode 88 isheld at ground potential at radio frequencies by means of bypasscapacitors 98 and 95. Also, one side of the heater filament for cathode85 is held at ground potential at radio frequencies by means of a bypasscapacitor 98, the other side of the filament being connected directly toground.

Referring again to the tuned line 8|, the ends of the outer conductorsof lines 82 and 83 are connected to ground through connections 93 and94. An adjustable shorting bar 96a/-96b is provided for tuning the twocircuits formed by each coaxial line and the center conductor 94 to themid-frequency of the received signal. It will be seen that there is oneresonant circuit that may be traced from the anode 18 through thecapacitor 84, the conductor 82, the shorting bar section 96a and theconductor 94 to ground; and a second resonant circuit that may be tracedfrom the anode 19 through the capacitor 81, the conductor 83, theshorting bar section 96h and the conductor 94 to ground.

A third resonant path is provided by a capacitor 91 that is inadjustable contact with the outer conductors of coaxial lines 82 and 83.This path may be traced from the anode 18 through the capacitor 84, theconductor 82, the capacitor 91, the conductor 83 through capacitor 81 tothe anode 19 and hence through the cathodes 88, 85. This third path ismade resonant at the midfrequency of the heterodyning or mixing signal,this mid-frequency being the same, of course, as the mid-frequency ofthe received reflected signal.

Referring now to the detector output circuit, it will be seen that thebeat frequency signal is taken off the anode 18 through the innerconductor of the coaxial line 82 and supplied through a conductor I8I, aresistor |82 and a portion of resistor |83 to a variable tap |84 and theoutput resistor 34. The beat frequency signal also is taken oif thecathode 85 through a low impedance filter resistor |88 (having aresistance of 100 ohms, for example) and supplied through a resistor |81and the other portion of resistor |03 to the variable tap |84 and theoutput resistor 34. A lter capacitor |88 is connected between ground andthe end of filter resistor |86 remote from the cathode. The resistor |85and the capacitor |88 are provided to keep the radio frequency out ofthe output circuit. The beat or audio frequency circuit for diode 11 iscompleted from the anode 19, through the inner conductor of coaxial line83 and through a connection |89 to the grounded conductor 94.

As Ain the circuit of Fig. 1, the mixing signal from the transmitter isapplied to the tuned circuit 8| in push-pull relation while the'reflected signal is applied thereto in parallel relation. This isaccomplished as follows: The mixing `signal is supplied to a loop II Iwhich, `as shown in Fig. 3, is positioned vcentrally with respect to thetuned circuit BI and with its plane parallel `to the plane of thecoaxial conductors 872 `and 83. The lines -of force from` loop II I willcut the lconductors 82 and 83 as indicated in Fig. 3 whereby the flow oftransmitter-or lmixing `signal in tuned circuit 3i will be as indicatedby the arrows fr in Fig. 2. Thus, the vmixing sig- 'nal at the anode 18is 180 degrees out of phase with the mixing signal at the anode '59.

The Yreflected signal is supplied through a co axial 4conductor H2 to aloop H3 which may be terminated by a tuning capacitor I I4. The loop II3 may be untuned like loop HI, if preferred. This loop is positionedcentrally with 'respect Vto tuned circuit BI and is located with itsplane at right angles to the plane of conductors 82 and 83. The linesYof force from loop `I I3 cut the conductors At2 and 83 as shown in Fig.8 whereby the flow of reected signal will beas shown by the arrows :inin Fig. 2. It will be seen Athat the reflected signal on the anode i8 isin 'phase with the reflected signal on the anode T9. The loops il! andAH3 are adjustable paralle] to the plane of the lines 82 and 33 torpermit balancing adjustments to compensate for any 'difference in diodecharacteristics or in the physical construction of the tuned circuit-82-83 or lother parts of Ithe receiver.

From the foregoing it will be evident that 4the operation of the circuit-of Fig. 2 is similar to that of Fig, l previously described, that is,any amplitude modulation may be balanced -out by adjusting the tap i055since the modulation at one anode `is in phase with that at the otheranode. The beat frequency signal, on the `other hand, is 180 degrees outof phase at one anode with respect to the beat frequency signal at theother anode whereby these signals add due to phase reversal obtained bythe output connections.

`In Fig. 3 electrostatic sheldslare shown at H5 and II'l `positioned`between the tuned line 32-53 andthe loops I I-I and H3, respectively.Actually,

these shields are not required if there is exact symmetry between thctwo loops and the line 82-8- The shields H6 and il? are indicatedschematically in Fig. 2. Also, in Fig. 2 a shieldingbox of aluminumorthe like for-the receiver is indicated at H8.

nIt may be noted, merely by way of example, Vthat all the capacitorsshown in Fig 2, except the tuning capacitor iid, have a capacity of `55micro-microfarads. Also, resistors -IilZ and |07 may be 5000 ohms each,resistor |03 may be 25,000 ohms and resistor 34' may be 47,000 ohms.These Values are for a receiver operating at a 440 lmegacycle carrierfrequency that is being swept through a range of 40 megacycles.

It should be understood that the mixing signal and the yreflected signalmay be applied to the detector in parallel relation and push-pullrelation, respectively, if desired instead of in the opposite waydescribed above.

It maybe noted that the invention may be utilized to advantage tomeasure the Doppler effect Yfor the purpose of determining the groundspeed of an-airplane for example. When the apparatus is so used, thefrequency modulator I2 is `disconnected from the transmitter so that aconstant frequency carrier wave is transmitted. In this case there willbe no amplitude modulation 4on the mixing signal but there may beundesired amplitude modulation on the reflected signal.

I claim as my invention:

11. A radio receiver comprising a pair of rectiers connected in balancedrelation, each rectier having an anode and a cathode, means 'for soapplying two carrier waves to said recntii-"lers that they have onephase relation with respect to each other at the anode of one rectinerandthe opposite phase relation with respect to Leach vother at the anodeof the other rectifier, 4a frequency responsive device, an outputimpedance unit `connected to supply signal to said frequency responsivedevice, means for taking the output signal of one rectifier off itsanode and means for vtaking the output signal of the other rectifier offits cathode, and means for supplying the signals from said anode andfrom said cathode to said output impedance unit whereby any amplitudemodulation on said carrier waves may be balanced out while supplyingsaid beat frequency signal to said frequency responsive device.

2. A radio system comprising means for transmitting a frequencymodulated carrier wave to a reflecting surface, a receiver system forreceiving said wave after reflection from said surface, said receivercomprising a pair of diodes connected in balanced relation, each diodehaving an anode and a cathode, means for applying said modulated wavedirectly from said transmitter to said diodes as a mixing signal wherebya difference or beat frequency signal is produced, said lreflected waveand said mixing signal having one phase relation with respect to eachother at one diode and the opposite yphase relation with respect to eachother at the other diode, a frequency responsive device, an output'Iimpedance unit connected to supply signal to said frequency responsivedevice, means for taking Vthe output signal of one diode off its anodeand means for taking the output signal of the other diode off itscathode, and means for supplying the signals from said anode and fromsaid cathode to said output impedance unit whereby any amplitudemodulation signal may be balanced out while supplying said beatfrequency signalto said frequency responsive device.

3. A radio distance determining system comprising means for transmittinga frequency modulated carrier wave to a reflecting surface, a receiverfor receiving said wave after reflection from said surface, saidreceiver comprising a pair of diodes connected in balanced relation,each diode having an anode and a cathode, Vmeans for applying saidreflected wave to said diodes and means for applying said modulated wavedirectly from said transmitter to said diodes whereby a difference orbeat frequency signal is produced, one or" said waves being applied tosaid diodes in push-pull relation and the 4other wave being applied tosaid diodes in parallel relation, and means for taking the resultingoutput signal of one diode off its anode and for taking the outputsignal of the other diode off its cathode, and means for adding saidoutput signals with such relative amplitudes that any amplitudemodulations of said output signals are substantially balanced outwhilethe beat frequency signal is passed to a utilization circuit.

'4. A radio distance determining system comprising means fortransmitting a frequency modulation carrier Wave to a reflectingsurface, a receiver system for receiving said wave after reflection fromsaid surface, saidreceiver comprising a pair of diode detector tubesconnected in balanced relation, means for applying said modulated wavedirectly from said transmitter to said detector tubes as a mixing signalwhereby a difference or beat frequency signal is produced, saidreflected wave and said mixing signal having one phase relation withrespect to each other at one detector tube and the opposite phaserelation at the other detector` tube, a frequency responsive device, andmeans for balancing out any amplitude modulation signal While supplyingsaid beat frequency signal to said frequency device, said meansincluding an output impedance unit, a connection from said impedanceunit to the anode of one of said diodes and a connection from saidimpedance unit to the cathode of the other of said diodes.

5. A radio receiver comprising a tuned line and a pair of heterodynedetector tubes connected in balanced relation thereto, means forapplying a first carrier Wave to said detector tubes and means forapplying a second carrier wave to said detector tubes whereby adiierence or beat frequency signal is produced, one of said waves beingapplied to said detector tubes in push-pull relation through a couplingloop positioned with its plane parallel to the plane of said tuned line,and the other Wave being applied to said detector tubes in parallelrelation through a coupling loop positioned with its plane at rightangles to the plane of said tuned line, and means for adding theresulting output signals of said detector tubes in such phase relationand with such relative amplitudes that any amplitude modulations of saidoutput signals are substantially balanced out while supplying said beatfrequency signal to a utilization circuit.

6. A radio system comprising means for transmitting a carrier wave to areflecting surface, a receiver for receiving said Wave after reflectionfrom said surface, said receiver comprising a tuned line and a pair ofheterodyne detector tubes connected in balanced relation thereto, meansfor applying said reected wave to said detector tubes and means forapplying said carrier wave directly from said transmitter to saiddetector tubes whereby a difference or beat frequency signal isproduced, one of said waves being applied to said detector tubes inpush-pull relation through a coupling loop positioned With its planeparallel to the plane of said tuned line, and the other wave beingapplied to said detector tubes in parallel relation through a couplingloop positioned with its plane at right angles to the plane of saidtuned line, and means for adding the resulting output signals of saiddetector tubes in such phase relation and with such relative amplitudesthat any amplitude modulations of said output signals are substantiallybalanced out while said beat frequency sig nal is passed to autilization circuit.

7. A radio receiver comprising a tuned line and a pair of diodesconnected in balanced relation thereto, each diode having a cathode andan anode, means for applying a rst carrier wave to said diodes and meansfor applying a second carrier wave to said diodes whereby a diiierenceor beat frequency signal is produced, one of said waves being applied tosaid diodes in push-pull relation through a coupling loop positionedWith its plane parallel to the plane of said tuned line,

and the other Wave being applied to said detec tor tubes in parallelrelation through a coupling loop positioned with its plane at rightangles to the plane of said tuned line, means for connecting the cathodeof one diode through a conductive connection to the low potential end ofsaid tuned line and for connecting the cathode of the other diodethrough a capacity connection to the low potential end of said tunedline, and means for adding the resulting output signals of said detectortubes in such phase relation and with such relative amplitudes that theamplitude modulations of said output signals are substantally balancedout, said last means comprising an output impedance unit, a conductiveconnection from the anode of said one diode to said impedance unit and aconductive connection from the cathode of said other diode to saidimpedance unit.

8. A radio receiver comprising a tuned line and a pair of diodesconnected in balanced relation thereto, each diode having a cathode andan anode, means for applying a rst carrier Wave to said diodes and meansfor applying a second carrier Wave to said diodes whereby a dilerence orbeat frequency signal is produced, one of said Waves being applied tosaid diodes in push-pull relation through a coupling loop positionedwith its plane parallel to the plane of said tuned line, and the otherwave being applied to said detector tubes in parallel relation through acoupling loop positioned with its plane at right angles to the plane ofsaid tuned line, said tuned circuit having an adjustable shorting barfor tuning it to the frequency of the carrier wave that is appliedthrough the right angle loop and having a capacitor connectedthereacross and adjustable along the line for tuning it to the frequencyof the carrier Wave that is applied through the parallel loop, means forconnecting the cathode of one diode through a conductive connection tothe low potential end of said tuned line and for connecting the cathodeof the other diode through a capacity connection to the low potentialend of said tuned line, and means for adding the resulting Outputsignals of said detector tubes in such phase relation and With suchrelative amplitudes that the amplitude modulations of said outputsignals are substantially balanced out, said last means comprising anoutput impedance unit, a conductive connection from the anode of saidone diode to said impedance unit and a conductive connection from thecathode of said other diode to said impedance unit.

9. The invention according to claim 8 wherein each side of said tunedline comprises a c0- axial line having an inner conductor and an outerconductor, said inner conductors being connected to the anodes of thediodes, said outer conductors having said shorting bar and said tuningcapacitor in adjustable contact therewith, and capacitors connectingeach end of said outer conductors to the corresponding end of its innerconductor.

ROYDEN C. SANDERS, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,268,587 Guanella Jan. 6, 19422,268,643 Crosby Jan. 6, 1942

